This invention relates generally to an X-ray mask structure for use in X-ray lithography. The invention further relates to an X-ray exposure method and apparatus and a semiconductor device manufacturing method which use such an X-ray mask structure. The invention still further relates to a semiconductor device to be produced on the basis of the semiconductor device manufacturing method.
In the stream of an increases in density and speed of a semiconductor integrated circuit, reduction in the linewidth of an integrated circuit pattern and improvement in the device manufacturing method have been required. In order to meet this, steppers as X-ray exposure apparatus using exposure light of an X-ray region (2-150 angstroms) have been developed.
X-ray mask structures for such X-ray exposure apparatus are usually such as shown in FIG. 1A. FIG. 1A is a sectional view of a conventional X-ray mask structure. As shown, the X-ray mask structure comprises at least an X-ray absorptive member 112, a supporting member 111 for supporting the absorptive member 112, and a holding frame 113 for holding the supporting member 111. There may be a reinforcing member 104 for reinforcing the holding frame 113 which may be adhered to the reinforcing member by an adhesive agent 105.
For mass production of semiconductor devices using an X-ray mask structure as shown in FIG. 1A, dust protection of the X-ray mask structure is inevitable. Generally, dust protection for an X-ray mask structure during exposures of wafers uses a thin film (pellicle) as a dust protection film. In the past, organic particles have a small influence on X-ray lithography, and no specific measure has been taken with respect to protection against dust at the dust protection film. However, with further reduction of a pattern to be produced by X-ray lithography, the presence of organic dust becomes influential to the exposure amount distribution, and it cannot be disregarded.
FIG. 1B is a sectional view of an X-ray mask structure with a thin film (pellicle) as a dust protection film (Japanese Laid-Open Patent Application, Laid-Open No. 72119/1988). In the X-ray mask structure shown in FIG. 1B, like that of FIG. 1A, an X-ray absorptive member 212 is supported by a supporting member 211 which is held by a holding frame 213. The frame 213 is adhered to a reinforcing member 204, for reinforcing the frame 213, by using an adhesive agent 205. A thin film (pellicle) 216 as a dust protection film is detachably fixed on the supporting film 211 through a frame 217. With this structure, the space at the wafer side of the supporting film 211 is covered by the thin film 216, whereby the supporting film 211 is protected against dust such as particles.
Another measure to provide dust protection may be cleaning an X-ray mask structure. However, because of a high aspect of the X-ray absorptive member of the X-ray mask structure, cleaning the X-ray mask structure is very difficult to accomplish. There may remain dust not removed by the cleaning. Further, since the supporting film for supporting the X-ray absorptive member is thin, its strength is low. The times of cleaning operations must be kept small. Additionally, while there is a high possibility that an electron beam is used for dust inspection, the dust inspection to be performed in the use of an X-ray mask structure may use light. However, it requires a very complicated process to discriminate dust and a pattern of high aspect. Also, in the stream of reduction in line width, a resist for pattern formation on a workpiece may use a chemical amplification type resist material. Depending on the resist used, a decomposition product may be created during the exposure process. Such a decomposition product may be adhered to the X-ray mask structure, which is a factor for causing a change with time of the X-ray mask structure.
When an X-ray mask structure with a pellicle is used in an exposure process (Japanese Laid-Open Patent Application, Laid-Open No. 72119/1988), there may be a problem in relation to alignment of the X-ray mask structure with the X-ray exposure apparatus or to alignment of a wafer to be exposed through the X-ray mask structure. This problem will be described below, with respect to an example of an optical heterodyne interference method used in an alignment detection system of a typical X-ray exposure apparatus.
FIG. 1C is a schematic view for explaining a process of detecting a phase difference between two beat signals on the basis of an optical heterodyne interference method (Japanese Published Patent Application, Publication No. 49926/1995). As shown in FIG. 1C, a supporting film 311 of an X-ray mask structure has a straight diffraction grating 320. On the face of the film 311 at the wafer 322 side, there is a thin film (pellicle) 316 which is fixed through a frame 317. On the top of the wafer 322, there is a straight diffraction grating 321 formed.
Alignment light 323 of a frequency f1 and alignment light 324 of a frequency f2 are projected and, in response, diffraction lights from the straight diffraction gratings 120 and 121 are caused to interfere with each other, whereby signal light 331 and signal light 332 are produced. On the basis of an optical heterodyne interference method, a phase difference between two beat signals is detected. Here, the thin film 316 reflects alignment light, such that noise light 333 is produced which goes along a path different from the signal light. The noise light 333 interferes with the signal light, causing a change in phase difference of beat signals. Because of this, it may become difficult to accurately detect a relative positional deviation of the X-ray mask structure and the wafer 322. As regards the light produced by reflection at the thin film 316, in addition to the noise light 333, there are plural noise lights coming along various light paths, reflected by the thin film 316. This causes adverse influence to detection of the positional deviation. Further, for a change in spacing (gap) between the X-ray mask structure and the wafer or any tilt of the mask structure to the wafer, a measured value of positional deviation may vary sensitively. Also, the intensity of alignment light passing through the pellicle of the X-ray mask structure may be reduced, and a sufficient signal level for keeping the precision for positional deviation detection may not be maintained.
This problem may apply to other alignment methods. In a method where a mask and a wafer are provided with grating lenses and a deflection angle of diffraction light by the grating lenses is measured, or in a method where images of marks of a mask and a wafer are detected, similarly, reflection of alignment light at a pellicle may produce noise light or a ghost image, causing lower alignment detection precision.
It is an object of the present invention to provide an X-ray mask structure by which, when alignment light is projected to the X-ray mask structure with a dust protection thin film, for detection of the position thereof, any positional deviation of the X-ray mask structure can be detected very precisely, such that the X-ray mask structure can be positioned very precisely.
It is another object of the present invention to provide an X-ray exposure method and apparatus using such an X-ray mask structure, by which high precision printing and mass production are enabled.